1
Modelling and Pe o mance Analysis o
Non-P ima y Channel Access in Wi-Fi Ne wo ks
Bo is Bellal a♭, F ancesc Wilhelmi♭, Lo enzo Gala i Gio dano⋆, and Gio anni Ge aci♯♭
♭Depa men o Enginee ing, Uni e si a Pompeu Fab a, Ba celona, Spain
⋆Radio Sys ems Resea ch, Nokia Bell Labs, S u ga , Ge many
♯Nokia S anda ds, Spain
Abs ac —This pape aims o imp o e ou unde s anding o
he pe o mance o he Non-P ima y Channel Access (NPCA)
mechanism, a new ea u e in oduced in IEEE 802.11bn o
enhance spec um u iliza ion in Wi-Fi ne wo ks. NPCA enables
de ices o con end o and ansmi on he seconda y channel
when he p ima y channel is occupied by ansmissions om an
O e lapping Basic Se ice Se (OBSS). We de elop a Con inuous-
Time Ma ko Chain (CTMC) model ha cap u es he in e -
ac ions among OBSSs in dense Wi eless Local A ea Ne wo k
(WLAN) en i onmen s when NPCA is enabled, inco po a ing
new NPCA-speci ic s a es and ansi ions. In addi ion o he
analy ical insigh s o e ed by he model, we conduc nume ical
e alua ions and simula ions o quan i y NPCA’s impac on
h oughpu and channel access delay ac oss a ious scena ios.
Ou esul s show ha NPCA can signi ican ly imp o e h ough-
pu and educe access delays in a o able condi ions o BSSs
ha suppo he mechanism. Mo eo e , NPCA helps mi iga e he
OBSS pe o mance anomaly, whe e low- a e OBSS ansmissions
deg ade ne wo k pe o mance o all nea by de ices. Howe e ,
we also obse e ade-o s: NPCA may inc ease con en ion on
seconda y channels, po en ially educing ansmission oppo u-
ni ies o BSSs ope a ing he e. O e all, he p oposed modeling
app oach o e s a ounda ion o analyzing, op imizing, and
guiding he de elopmen o NPCA in nex -gene a ion Wi-Fi
ne wo ks.
Index Te ms—Non-p ima y channel access, NPCA, IEEE
802.11bn, WLAN, Wi-Fi 8.
I. INTRODUCTION
The con ex : Wi-Fi channel wid hs ha e inc eased o e ime,
om he ini ial 20 MHz o 320 MHz, going h ough 40, 80,
and 160 MHz. The adop ion o wide channels aims o en-
hance ne wo k pe o mance by inc easing ansmission a es,
imp o ing h oughpu , and educing la ency [1]. Howe e , as a
esul o cu en channel sensing ules based on Lis en Be o e
Talk (LBT), his comes wi h he d awback o highe con en ion
wi h neighbo ing ne wo ks ope a ing on he same channel o
o e lapping po ions o i [2].
Channels wide han 20 MHz consis o a manda o y 20
MHz p ima y channel—whe e all de ices wi hin he same
Basic Se ice Se (BSS) pe o m channel access con en ion
(e.g., backo coun down)—and one o mo e seconda y chan-
nels. Seconda y channels can only be used i hey a e idle a
he ime a ansmission oppo uni y is secu ed ia he p ima y
channel. I one o mo e seconda y channels a e occupied by
an O e lapping BSS (OBSS), hey a e e ec i ely bypassed
by punc u ing hem hanks o he p eamble punc u ing ea u e
in oduced in IEEE 802.11ax (Wi-Fi 6) [3]. While his educes
he e ec i e channel wid h, i s ill allows de ices o ansmi
using he ins an aneous a ailable spec um.
The p oblem: Wi-Fi cu en ly lacks a solu ion o scena ios
whe e he p ima y channel o a BSS is occupied by an
OBSS ansmission. In such cases, e en i all he seconda y
channels a e a ailable, de ices in he a ec ed BSS mus de e
hei ansmissions un il he p ima y channel becomes idle
again. This esul s in educed channel access oppo uni ies and
ine icien spec um u iliza ion, as illus a ed in Fig. 1 (uppe
pa ).
The 802.11bn solu ion: To add ess his issue, IEEE
802.11bn [4], [5] is de eloping a new ea u e called Non-
P ima y Channel Access (NPCA). The concep is s aigh -
o wa d: when a BSS de ec s an OBSS ansmission on i s
p ima y channel, NPCA-complian de ices empo a ily swi ch
o a seconda y channel, designa ed as he NPCA p ima y chan-
nel, o pe o m channel con en ion and ini ia e ansmissions
while he p ima y is busy. This enables he BSS o u ilize idle
seconda y channels e ec i ely. The bene i s o his app oach
a e illus a ed in Fig. 1 (lowe pa ). Wi h NPCA, he a ge
BSS inc eases i s chances o accessing he channel, which
is c ucial o la ency-sensi i e a ic, while ensu ing he ull
u iliza ion o a ailable spec um.
Ou con ibu ion: In his pape , we in es iga e he po en-
ial h oughpu and channel access delay gains o NPCA.
To achie e his, we use Con inuous Time Ma ko Chains
(CTMCs) o model he sys em beha io wi h and wi hou
NPCA. CTMCs ha e been widely and success ully applied o
analyze he pe o mance o complex Ca ie Sense Mul iple
Access wi h Collision A oidance (CSMA/CA) ne wo ks [6]–
[8], including scena ios in ol ing ad anced IEEE 802.11 ea-
u es such as channel bonding [9]–[11] and spa ial euse [12],
[13]. Speci ically, we eso o CTMC models o cap u e
NPCA ope a ion, le e aging hei sui abili y o s udying
CSMA/CA-based ne wo ks in gene al and Wi-Fi in pa icula .
The main con ibu ions o his pape a e as ollows:
1) We desc ibe he p inciples o NPCA ope a ion based
on IEEE 802.11bn discussions, and p opose o suppo
mul i-NPCA consecu i e ansmissions, aiming o be e
exploi la ge NPCA oppo uni ies.
2) We ex end CTMC 802.11 models o inco po a e NPCA
2
Fig. 1: Legacy ( op) s. NPCA (bo om) ope a ion, whe e ‘P’
and ‘S’ deno e he p ima y and seconda y channel, espec-
i ely, e.g., an 80 MHz channel each. The igu e illus a es
how NPCA enables nea -con inuous ansmissions, esul ing
in highe h oughpu and educed channel access delays, as
e lec ed by he lowe imes be ween consecu i e channel
accesses (τ alues).
ope a ion, enabling de ailed pe o mance analysis. As
pa o ou modeling app oach, we examine he key
design conside a ions and unde lying assump ions, de-
ailing hei implica ions and how hey in luence he
achie able esul s and hei signi icance, he eby laying
he g oundwo k o u u e modeling e o s.
3) We use he CTMC model o e alua e NPCA and demon-
s a e ha i e ec i ely p o ides h oughpu and channel
access gains o he NPCA-enabled BSS while emain-
ing comple ely anspa en o OBSS ne wo ks ope a ing
on he NPCA-enabled BSS p ima y channel.
4) We unco e he ela ionship be ween he T ansmission
Oppo uni y (TXOP) du a ion and NPCA pe o mance,
highligh ing a ade-o be ween NPCA h oughpu gains
and he Agg ega ed MAC P o ocol Da a Uni (A-
MPDU) size.
5) We show ha NPCA mi iga es he OBSS pe o mance
anomaly, pa icula ly when OBSS ansmissions a e
disp opo iona ely long due o using low Modula ion and
Coding Scheme (MCS), while he a ge BSS employs
high MCSs. In such cases, h oughpu gains exceeding
×2can be achie ed.
6) We illus a e ha , in he p esence o OBSSs ope a ing
on he seconda y channel o he a ge NPCA BSS, en-
abling NPCA has a signi ican impac on he h oughpu
dis ibu ion be ween he a ge BSS and he OBSSs.
II. NON PRIMARY CHANNEL ACCESS
The ope a ion o NPCA is illus a ed in Fig. 2, whe e
he ac i i y on p ima y and seconda y channels o a gi en
BSS is shown. In he con ex o NPCA, we e e o he
seconda y channel as he NPCA channel. Addi ionally, he
20 MHz channel wi hin he NPCA channel used o con en ion
is e e ed o as he NPCA p ima y channel.
When an OBSS ansmission is de ec ed on an NPCA-
enabled BSS’s p ima y channel, a e ecei ing bo h Reques -
To-Send (RTS) and Clea -To-Send (CTS) ames, all NPCA-
capable de ices swi ch hei con ol channel o a p ede ined 20
MHz seconda y channel (i.e., he NPCA p ima y channel). The
con en ion in he NPCA p ima y channel begins a e a delay
o TNPCA om he s a o he OBSS ansmission. This delay
TNPCA co esponds o he du a ion o he RTS/CTS exchange.
Once he backo coun e eaches ze o, a ansmission on he
seconda y channel is ini ia ed ollowing he de aul 802.11
ules.
A key equi emen o NPCA is ha ansmissions mus
comple e be o e he OBSS ansmission ends, so ha he
BSS can esume ope a ion on i s de aul p ima y channel.
This cons ain is impo an because no all de ices wi hin
he BSS a e assumed o suppo NPCA. We also accoun o a
swi ching delay, deno ed Tswi ch, which ep esen s he ime
equi ed o e u n om he NPCA p ima y channel o he
de aul p ima y channel. As a esul , he ime a ailable o
an NPCA ansmission oppo uni y is limi ed by he du a ion
o he OBSS ansmission, educed by he sum o TNPCA
and Tswi ch. These addi ional swi ching o e heads educe he
e ec i e ime a ailable o da a ansmission in NPCA, leading
o sligh ly lowe e iciency compa ed o legacy (non-NPCA)
ansmissions.
Finally, we make he ollowing wo assump ions ega ding
NPCA ope a ion, bo h o which le e age he ac ha he
NPCA BSS is awa e o he expec ed end ime o he OBSS
ansmission occupying i s p ima y channel:
1) A e swi ching o he NPCA p ima y channel due o
de ec ing an OBSS ansmission on he p ima y channel,
i he NPCA p ima y channel is ini ially busy, he a ge
BSS will con inue o moni o i and a emp o ansmi
i i becomes idle be o e he NPCA oppo uni y ends.
2) I a ansmission on he NPCA channel concludes and
he e is s ill su icien ime o ini ia e ano he ansmis-
sion, he a ge BSS will e-con end o access on i
by execu ing a new backo p ocedu e on he NPCA
p ima y channel. I success ul, i will ini ia e ano he
ansmission, wi h i s du a ion adjus ed o i wi hin he
emaining ime o he OBSS ansmission.
In bo h cases, he a ge BSS e u ns o i s de aul p ima y
channel be o e he OBSS ansmission comple es.
III. SYSTEM MODEL
To analyze he NPCA pe o mance, we conside he Wi-
Fi deploymen depic ed in Fig. 3. The scena io consis s o
ou BSSs, each consis ing o one Access Poin (AP) and
one s a ion (STA). The channel alloca ion o each BSS
is de ailed in he igu e: BSS A and C u ilize 160 MHz
channels, while BSS B and D ope a e on 80 MHz channels.
All de ices, including APs and STAs, a e wi hin each o he ’s
co e age a ea, elimina ing he possibili y o hidden e minals.
Fo simplici y, he lowe 80 MHz o he 160 MHz channel
is e e ed o as channel 1 (Ch#1), and he uppe 80 MHz
as channel 2 (Ch#2). When APs A and C use he 160 MHz
channel, i is indica ed as u ilizing bo h Ch#1 and Ch#2.
STAs a e deployed a a dis ance d om hei espec i e
APs. The TMB pa h loss model o he 5 GHz band in
indoo o ice en i onmen s is conside ed [14]. The selec ed
3
Fig. 2: Non P ima y Channel Access ope a ion.
Fig. 3: Fou o e lapping BSSs.
MCS alues depend on he ecei ed powe and ange om
MCS 11 (1024-QAM; 5/6) o MCS 1 (BPSK; 1/2). We use
di e en ansmission powe alues when ansmi ing o e
80 MHz (20 dBm) and 160 MHz (23 dBm) channels o
ensu e ha he same MCS is used ega dless o he channel
wid h. The maximum TXOP du a ion is se o Tmax = 5
ms, encompassing he RTS/CTS exchange, da a ansmission,
Block Acknowledgmen (BACK), and in e - ame spaces. A
Packe E o Ra e (PER) o 0.1is assumed o all MCSs.
All ansmissions u ilize wo spa ial s eams o he da a pa ,
while one spa ial s eam is used o all con ol ames.
Only downlink a ic is conside ed, meaning ha only
APs ansmi da a packe s. Th oughou he pape , we use he
e ms AP and BSS in e changeably o e e o he ansmi ing
de ice. Each da a packe has a size o Lbi s. A-MPDU
packe agg ega ion is employed, wi h Npacke s agg ega ed
pe ansmission. The alue o Ndepends on he MCS,
channel wid h, he maximum TXOP du a ion Tmax, and he
maximum A-MPDU size ∆. Speci ically, N≤min(M, ∆),
whe e Mis he maximum numbe o packe s ha can be
ansmi ed wi hin Tmax o a gi en MCS and channel wid h.
Fo example, using MCS 11, BSS A can ansmi up o 968
da a packe s o 1400 by es each (app oxima ely 10 Mbi s)
wi hin Tmax on a 160 MHz channel. In con as , wi h MCS 1
on an 80 MHz channel, BSS A can ansmi up o 29 packe s
(app oxima ely 325 kbi s) du ing he same du a ion.
Rega ding NPCA ope a ion, we assume ha a backo
ins ance is gene a ed o each ansmission, ega dless o
whe he he BSS ope a es in legacy o NPCA mode, ollowing
he de aul IEEE 802.11 ules. In legacy mode, he backo
coun e is paused whene e he p ima y channel is ound o be
busy, whe eas in NPCA mode, i is paused only when bo h he
p ima y and seconda y channels a e busy. No e ha a BSS may
swi ch be ween legacy and NPCA modes mul iple imes du ing
a single backo ins ance, un il channel access is achie ed.
We s udy he pe o mance o NPCA in h ee ep esen a i e
scena ios ha a e ex ac ed om he deploymen o Fig. 3.
These h ee scena ios co e he mos impo an aspec s needed
o s udy NPCA pe o mance. Conside ing mo e BSSs will
only inc ease con en ion, p opo ionally scaling he pe o -
mance each BSS can achie e, wi hou p o iding u he in-
sigh s. Resul s in o he scena ios can be easily ex apola ed
om he ones p esen ed in his pape .
1) Scena io I: BSSs A and B a e ac i e. This scena io p o-
ides he mos a o able condi ions o e alua e NPCA
h oughpu gains o BSS A, as i s 80 MHz seconda y
channel emains always a ailable. NPCA is expec ed
o inc ease BSS A’s channel access a e, enhancing
h oughpu and educing access delay.
2) Scena io II: BSSs A, B, and D a e ac i e. Compa ed
o Scena io I, ac i a ing BSS D occupies BSS A’s
seconda y 80 MHz channel, limi ing NPCA ansmission
oppo uni ies. This scena io examines how ac i i y on
BSS A’s seconda y channel impac s NPCA gains.
3) Scena io III: All ou BSSs a e ac i e. This c ea es a
symme ic scena io whe e BSS A and BSS C u ilize
NPCA o access channels 2 and 1, espec i ely. We
in es iga e whe he NPCA s ill p o ides pe o mance
gains in such a balanced se ing.
In Table I, we p esen he pa ame e s used in he pe o -
mance e alua ion. Unspeci ied pa ame e s ollow he IEEE
802.11 speci ica ions. I di e en alues a e conside ed du ing
he e alua ion, hey will be indica ed acco dingly. Fo each
da a poin in he esul s, 500 andom ins ances o a gi en
scena io ha e been simula ed, wi h each ins ance las ing 10 s.
IV. MODELING NPCA WITH CTMCS
CTMC models a e widely adop ed o hei abili y o e ec-
i ely cap u e he complex, asynch onous in e ac ions among
de ices sha ing spec um esou ces h ough CSMA/CA, as in
4
Pa ame e Value Pa ame e Value
CWmin 16 L 1400 By es
dU[1,17] me e s MCSs (11ax) [1-11]
P x (80 MHz) 20 dBm P x (160 MHz) 23 dBm
Tmax 5 ms Num SS. 2
∆[1-1024] PER 0.1
TNPCA 0.136 ms Tswi ch 16 µs
OFDM symbol 13.6 µsBacko slo 9 µs
DIFS 34 µs SIFS 16 µs
Leg. PHY p eam. 20 µs PHY p eam. 100 µs
RTS 160 bi s CTS 112 bi s
MAC heade 240 bi s BACK 240 bi s
MPDU Del. 32 bi s Tail Bi s 18 bi s
TABLE I: Value o he pa ame e s used in he pe o mance
e alua ion.
Wi-Fi ne wo ks. These models ha e been alida ed agains
simula ions in [7], [9]–[12], [15]–[18], demons a ing hei
accu acy, ep esen a i eness, and consis ency.
A CTMC model cap u es he sys em’s dynamics, enabling
he analysis o i s s eady-s a e pe o mance [6]. In apply-
ing CTMCs o cha ac e ize Wi-Fi, we conside ha a s a e
s∈ ⊗—whe e ⊗is he se o all CTMC s a es—is de ined as
he se o ac i e BSSs (i.e., BSSs ha a e concu en ly ans-
mi ing), and we assume ha channel access con en ion and
ansmission du a ions a e go e ned by s ochas ic p ocesses
ha ollow exponen ial dis ibu ions.
Howe e , CTMCs canno accu a ely model collisions be-
ween con ending de ices. Speci ically, because backo du a-
ions a e assumed o be exponen ially dis ibu ed, he p obabil-
i y ha wo de ices comple e hei backo a exac ly he same
ime is ze o. As a esul , simul aneous channel access—and
hence collisions—a e e ec i ely excluded om he model. In
ou scena io, he impac o neglec ing collisions is minimal, as
he numbe o con ende s is small— esul ing in a low collision
p obabili y by de aul [19]—and he use o RTS/CTS u he
minimizes collision du a ion, which is app oxima ely 30 imes
sho e han ha o a success ul ansmission. Consequen ly,
neglec ing collisions does no signi ican ly a ec he o e all
sys em pe o mance (see Sec ion IV-D). None heless, he
impac o collisions can be inco po a ed in CTMCs-based
models using he app oach ou lined in [15].
The s a iona y dis ibu ion (π) o a CTMC is de i ed by
sol ing πQ= 0, whe e Qis he in ini esimal gene a o ma ix
o he s ochas ic p ocess. Each elemen o Q, deno ed Qi,j,
ep esen s he ansi ion a e om s a e i o s a e j. Fo wa d
ansi ions (e.g., ini ia ing a ansmission) occu a a e λ,
while backwa d ansi ions (e.g., comple ing a ansmission)
occu a a e µ.
A. CTMC S a es
To model ou desc ibed scena io, which includes NPCA
ansmissions, we adop he app oach used in [10], [11] o
analyzing Dynamic Channel Bonding (DCB). Howe e , in his
wo k, we in oduce a new ype o s a e, e e ed o as an NPCA
s a e, whe e NPCA ansmissions occu . A key cha ac e is ic
o NPCA s a es is ha , once he OBSS ansmission ends,
he sys em ansi ions o he same subsequen s a e ha would
ollow he OBSS ansmission in he absence o NPCA.
NPCA s a es become easible when a BSS suppo ing
NPCA de ec s ha i s p ima y channel is occupied by an OBSS
ansmission. I he seconda y channel is idle, he NPCA-
enabled BSS ini ia es (a e con ending) a ansmission on ha
seconda y channel, ollowing he NPCA ope a ion desc ibed in
Sec ion II. The only di e ence in ou model is ha con en ion
on he seconda y channel is assumed o begin immedia ely
a e de ec ing he p ima y channel is busy, a he han a e
wai ing o TNPCA seconds. In such a si ua ion, since he
channel access p obabili ies o bo h he NPCA BSS and he
OBSSs depend solely on hei backo pa ame e s, his may
esul in highe h oughpu o he NPCA BSS a he expense
o educed h oughpu o he OBSSs, pa icula ly i all o
hem use he same backo con igu a ion.
Fig. 4 illus a es he CTMC o he Wi-Fi deploymen
depic ed in Fig. 3, compa ing wo scena ios: (a) when NPCA is
no suppo ed (o disabled), and (b) when NPCA is enabled.
Each s a e is iden i ied by he BSSs ansmi ing simul ane-
ously ( he capi al le e s), along wi h he speci ic channels
each one uses (indica ed by subsc ip s). Fo example, s a e
A12 deno es BSS A ansmi ing alone o e Ch#1 and 2 (a
160 MHz ansmission), while s a e B1D2indica es ha BSS
B and BSS D a e ansmi ing concu en ly—BSS B using
Ch#1 and BSS D using Ch#2, each pe o ming 80 MHz
ansmissions. S a es ou lined wi h dashed lines co espond
o Dynamic Channel Bonding (DCB) ope a ions. As de ailed
in [10], [11], hese s a es a ise when a BSS ansmi s o e
only a po ion o i s alloca ed channel. DCB s a es canno be
eached om he idle s a e (s a e 0), since a BSS will no
olun a ily access jus a ac ion o i s alloca ed bandwid h
when he en i e channel is a ailable. S a es in which only BSSs
A and B a e ac i e a e highligh ed in yellow, co esponding o
he ac i e s a es o Scena io I desc ibed in Sec ion III. Wi hou
NPCA (Fig. 4.a), when BSS B (o BSS D) accesses Ch#1 (o
Ch#2), he o he channel can only be used by BSSs C and D
(o BSSs A and B, espec i ely). Fo ins ance, om s a e B1,
he sys em can only ansi ion o s a es B1C2o B1D2. When
NPCA is enabled (Fig. 4.b), wo addi ional s a es become
a ailable: B1A∗
2and D2C∗
1. These s a es ep esen scena ios
whe e BSS A o BSS C ini ia es a ansmission on hei NPCA
channel while hei espec i e p ima y channel is occupied by
BSS B o BSS D. T ansi ions om hese NPCA s a es e u n
o s a e 0upon he comple ion o he OBSS ansmission ha
ini ially blocked access on he p ima y channel. As explained
in Sec ion II, du ing his in e al, mul iple consecu i e NPCA
ansmissions may occu i he NPCA-enabled BSS (e.g.,
BSS A) comple es i s ansmission quickly bu s ill has packe s
queued in i s bu e .
The CTMC s a es e eal ha enabling NPCA inc eases
con en ion, as illus a ed in Fig. 4b wi h he highe numbe
o o wa d ansi ions om s a es B1and D2compa ed o he
same s a es in Fig. 4a, which en ail ha a bigge se o con-
ende s is compe ing o he channel. The addi ional con en ion
in oduced by NPCA can be mi iga ed by con igu ing NPCA
ansmissions o use mo e conse a i e backo pa ame e s,
such as a la ge con en ion window (CW). Howe e , his
ques ion lies beyond he scope o his pape .
5
Fig. 4: CTMC modeling he Wi-Fi deploymen shown in Fig. 3 when a) Legacy (NPCA disabled), and b) NPCA is enabled.
B. CTMC T ansi ions
1) Channel access a e, λ:The ansmission a emp a e,
i.e., how agg essi ely a de ice con ends o access he channel,
is de ined as he ecip ocal o he expec ed backo ime, gi en
by
λ=1
E[backo ]=2
(CW −1)Te
,(1)
whe e Teis he du a ion o an emp y slo (9 µs).
2) T ansmission a e, µ:The mean ansmission du a ion
E[Ts]=1/µ ep esen s he ime a de ice occupies he channel
a e gaining access. We ollow he ames and in e als
speci ied by he IEEE 802.11 p o ocol. In pa icula , he
du a ion o an A-MPDU ansmission comp ising Npacke s
is gi en by:
Ts=TRTS + 3 ·TSIFS +TCTS +TDATA +TBACK +TDIFS +Te,
(2)
whe e
TDATA =TPHY +LH+N(LD+L) + LT
DBPS TOFDM,(3)
wi h LHas he MAC heade , LDas he MPDU delimi e ,
Las he MPDU size, and LTas he ail bi s. DBPS deno es
he da a bi s pe symbol, which depends on he numbe o
subca ie s (and he e o e o he channel wid h), he numbe
o spa ial s eams, and he MCS used.
Fo NPCA ansmissions, he maximum ansmission du a-
ion is de e mined by he OBSS ansmission du a ion, i.e.,
Ts,OBSS, and includes addi ional o e heads: he ime equi ed
o swi ch o he NPCA p ima y channel (TNPCA) and o e u n
o he o iginal p ima y channel (Tswi ch). Consequen ly, he e -
ec i e ansmission ime o NPCA is Ts,OBSS−TNPCA−Tswi ch.
In bo h legacy (i.e., NPCA disabled) and NPCA ansmis-
sions, he numbe o packe s ansmi ed is calcula ed as he
maximum numbe o packe s ha can be agg ega ed wi hin he
a ailable ime, limi ed by he maximum A-MPDU size (∆) as
desc ibed in Sec ion IV.
C. Pe o mance Me ics
In his pape , we conside wo pe o mance me ics as
desc ibed nex .
Th oughpu (bps): The h oughpu Γ(n)o BSS nis de ined
as
Γn= (1 −PER) X
∀s∈Ω:n∈s
µs
nNs
nπs!L, (4)
whe e µs
nNs
nLis he amoun o da a bi s/second e ec i ely
ansmi ed when he sys em is in s a e sby BSS n. The PER
is applied as a scaling ac o .
Channel access delay (ms): To es ima e he channel access
delay, we pe o m an e en -based simula ion based on he Q
ma ix. S a ing om he cu en s a e, we iden i y he ea lies
upcoming e en and ansi ion o he co esponding nex
s a e, upda ing he sys em ime acco dingly. Th oughou he
simula ion, we eco d he ime in e als be ween consecu i e
ansmissions o each BSS. These in e als e lec bo h he
6
p obabili y o success ul channel access and he ime spen
de e ing due o con en ion, encompassing he o al ime a
BSS spends de e ing and ansmi ing. All simula ions begin
om s a e 0.
No e ha he channel access delay esul s ob ained om
he CTMC model inhe en ly cap u e he a iabili y o bo h
backo and ansmission du a ions, which ollow exponen ial
dis ibu ions. The e o e, in Sec ion V, we epo only he mean
channel access delay alues, as hey acili a e he in e p e a ion
o he esul s, and p o ided insigh s.
D. Valida ion
To alida e he CTMC model, we compa e i s h oughpu
and channel access delay esul s wi h hose ob ained om an
IEEE 802.11 simula o 1, which has been ex ended o suppo
NPCA as desc ibed in Sec ion III. The scena io pa ame e s
used o nume ical e alua ion a e as ollows: dA= 1.5m
(MCS 11), dB= 17 m (MCS 1), dC= 5 m (MCS 6), and
dD= 5 m (MCS 6). The maximum packe agg ega ion is se
o 128 packe s. Each simula ion un las s 50 seconds, and he
esul s combine i e independen uns using di e en andom
numbe gene a o seeds, o a o al simula ion ime o 200 s.
The simula o implemen s he s anda d IEEE 802.11 bina y
exponen ial backo wi h CWmin = 15 and CWmax = 1024.
T ansmission du a ions a e de e minis ic and depend solely on
he channel wid h used and he numbe o ixed-leng h packe s
included.
Table II p esen s he esul s unde legacy ope a ion (i.e.,
when NPCA is disabled). Unde hese condi ions, he CTMC
model closely ma ches he simula ion esul s, wi h only mino
disc epancies. These a e p ima ily a ibu ed o collisions oc-
cu ing in he simula o , which a e no cap u ed by he CTMC
model. Collision a es obse ed in he simula ion a e consis en
wi h hose p edic ed by Bianchi’s model [19], which es ima es
a collision p obabili y o app oxima ely 0.11 when wo ull-
bu e APs con end o channel access. I is wo h no ing
ha in he simula ion, he channel access delay includes he
ime spen in collisions, as i is measu ed om he momen a
ansmission is scheduled un il he co esponding Block ACK
is ecei ed.
Table III p esen s he esul s when NPCA is enabled. O e -
all, bo h h oughpu and channel access delay me ics om he
model and simula ions emain highly accu a e. Howe e , some
small bu no ewo hy disc epancies a ise. Speci ically, BSSs A
and C (NPCA-capable) achie e sligh ly lowe h oughpu
han expec ed, while BSSs B and D (non-NPCA-capable)
expe ience sligh ly highe h oughpu han an icipa ed. Al-
hough hese di e ences a e mino , hey e lec an in e es ing
beha io oo ed no in collisions only, bu in he simula o ’s
implemen a ion o a single backo ins ance pe BSS, which
con inues unning as he BSS swi ches be ween legacy and
NPCA modes. In de ail, his single-backo implemen a ion
in oduces wo con as ing e ec s o NPCA-capable BSSs.
1The NPCA simula o is based on he Komondo Wi-Fi simula o : h ps:
//gi hub.com/wn-up /Komondo
Fi s , i educes hei chances o accessing hei p ima y
channel. When swi ching om NPCA back o legacy mode,
BSSs A and C mus d aw a new backo , whe eas non-NPCA
BSSs B and D may esume a paused backo , gaining an
ad an age. Fo ins ance, in Scena io I, BSS B pauses i s
backo du ing BSS A’s ansmissions, esul ing in a sho e
a e age backo o subsequen access a emp s. Con e sely,
since BSS A ese s i s backo a e NPCA use (happening
du ing BSS B’s ansmissions), i is less likely o win nex
con en ion agains BSS B on hei p ima y channel, which
also limi s he numbe o 160 MHz ansmissions i can
pe o m. A he same ime, e aining he same backo ins ance
bene i s NPCA access. When BSSs A and C swi ch o he
NPCA channel, hey con inue using he backo d awn on
he p ima y channel, o en esul ing in quicke access. In
Scena io II, BSS D loses mo e con en ions o BSS A’s NPCA
ansmissions, sligh ly educing i s h oughpu . Howe e , hese
addi ional NPCA oppo uni ies only pa ially o se he loss in
legacy ansmissions, explaining he obse ed di e ences.
While he esul s p esen ed in his sec ion a e p ima ily
in ended o alida e he model’s use ulness in analyzing NPCA
pe o mance, hey also highligh impo an conside a ions o
he u u e de elopmen o NPCA. In pa icula , hese indings
sugges he need o ca e ul design o backo policies in
NPCA ope a ion. Speci ically, i is necessa y o de ine how
backo coun e s should be managed when swi ching be ween
legacy and NPCA modes. This includes de e mining whe he
a single backo ins ance should be main ained ac oss bo h
modes, o i sepa a e backo s should be used o each, along
wi h clea ules o ese ing coun e s and adjus ing con en ion
windows in esponse o collisions. These ques ions challenge
he con en ional link be ween indi idual ansmissions and
hei associa ed backo p ocesses. Fo ins ance, i a colli-
sion occu s du ing legacy access bu an NPCA oppo uni y
becomes a ailable immedia ely a e wa d, should he sys em
double he backo on he NPCA channel? O should a new
backo be d awn? Such decisions could signi ican ly impac
ai ness and e iciency, and hus equi e ca e ul conside a ion
in u u e NPCA p o ocol design.
O e all, he esul s con i m ha he CTMC model p o ides
accu a e pe o mance es ima es—bo h quan i a i ely (in e ms
o h oughpu and channel access delay) and quali a i ely (in
cap u ing he in e ac ion dynamics among con ending BSSs).
V. PERFORMANCE EVALUATION
In his sec ion, we e alua e he pe o mance o NPCA in
he scena ios ou lined in Sec ion III. We p esen insigh s in o
how NPCA imp o es Wi-Fi ne wo k h oughpu and educes
channel access delay, and analyze how i s ac i a ion in lu-
ences spec um sha ing and in e ac ion among o e lapping
BSSs (OBSSs).
A. NPCA Th oughpu and Channel Access Delay Gains
We s udy he NPCA h oughpu and delay gains in Sce-
na io I, whe e he seconda y 80 MHz channel o BSS A—i s
NPCA channel—is always a ailable. In his scena io, when
NPCA is enabled, BSS A is able o access i s seconda y
7
CTMC model Simula ion
Scena io I Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 213.9 6.05 211.6 6.09 0.1087
BSS B 48.5 5.98 48.12 6.07 0.1084
Scena io II Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 194.9 6.65 193.3 6.67 0.110
BSS B 44.1 6.55 43.8 6.66 0.109
BSS D 475.0 2.70 473.5 2.72 0.000504
Scena io III Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 193.6 6.68 191.9 6.72 0.111
BSS B 43.8 6.72 43.5 6.72 0.110
BSS C 241.9 5.39 238.9 5.40 0.112
BSS D 241.9 5.41 240.4 5.37 0.111
TABLE II: CTMC model s simula ion esul s - Legacy ope a ion (Wi hou NPCA)
CTMC model Simula ion
Scena io I Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 850.7 1.23 768.0 1.66 0.030
BSS B 48.5 5.99 50.22 5.72 0.104
Scena io II Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 375.4 2.93 369.4 3.12 0.125
BSS B 44.74 6.70 45.37 6.29 0.113
BSS D 360.7 3.53 338.3 3.69 0.092
Scena io III Th oughpu (Mbps) Ch. Access Delay (msecs) Th oughpu (Mbps) Ch. Access Delay (msecs) Coll. P ob
BSS A 277.7 4.31 268.7 3.81 0.237
BSS B 39.7 7.33 39.53 6.89 0.203
BSS C 245.0 4.53 228.1 4.49 0.258
BSS D 212.4 6.09 210.1 5.32 0.229
TABLE III: CTMC model s simula ion esul s - NPCA enabled
channel (Ch#2) when i s p ima y channel (Ch#1) is busy,
hence ob aining bo h highe h oughpu and lowe channel
access delay.
Th oughpu and channel access delay dis ibu ions a e de-
i ed om mul iple scena io ins ances. In each ins ance, we
andomize wo pa ame e s pe BSS: i) he s a ion posi ions,
placing hem a andom dis ances uni o mly dis ibu ed be-
ween 1 and 17 me e s om hei co esponding AP, wi h
MCSs assigned acco dingly, and ii) he maximum A-MPDU
alue (∆), anging om 1 o i s uppe limi (1024), o
in oduce high a iabili y in ansmission sizes and du a ions.
Fo e e ence, in Scena io I, when BSS A uses MCS 11
(1024-QAM, coding a e 5/6) o e a 160 MHz channel, i can
ansmi A-MPDUs o up o 968 packe s (each 1400 by es)
wi hin a single TXOP, cons ained by he maximum TXOP
du a ion (Tmax = 5 ms). Simila ly, bo h BSSs A and B, using
MCS 11 on an 80 MHz channel, can ansmi up o 484 packe s
pe TXOP.
Fig. 5a shows he h oughpu dis ibu ion as a boxplo 2 o
BSSs A and B wi hou and wi h NPCA enabled. Wi hou
NPCA, since BSS A and BSS B sha e he same p ima y
channel and use he same CWmin, ollowing CSMA/CA
ope a ion hey access he channel he same numbe o imes
on a e age. The e o e, he highe h oughpu achie ed by
BSS A is only because i is able o ansmi using he ull
160 MHz channel e e y ime i accesses he channel, which
u ns ou on mo e packe s ansmi ed pe TXOP. Enabling
2A boxplo shows he median ( he line inside he box), he i s qua -
ile (25 h pe cen ile) and hi d qua ile (75 h pe cen ile) as he bo om and op
edges o he box, espec i ely. The ”whiske s” ex end o he mos ex eme
da a poin s wi hin 1.5 imes he in e qua ile ange (IQR) om he qua iles,
ep esen ing he minimum and maximum alues wi hin ha ange. Poin s
ou side his ange a e ypically plo ed indi idually as ou lie s.
(a) Th oughpu dis ibu ion.
(b) Mean Channel Access Delay.
Fig. 5: Th oughpu and channel access delay esul s ob ained
in Scena io I.
NPCA inc eases he h oughpu o BSS A by a ac o o
≈ ×1.5, as i can now access i s seconda y 80 MHz channel
(Ch#2) while BSS B is occupying i s p ima y 80 MHz channel
(Ch#1), whe eas be o e i had o de e . As expec ed, BSS B
is una ec ed by BSS A’s NPCA ansmissions—which occu
simul aneously bu on a di e en channel—and he e o e, i s
8
h oughpu dis ibu ion emains unchanged. Ou lie s ep esen
possible bu low-p obabili y h oughpu alues, as hey depend
on speci ic con igu a ion and scena io pa ame e s o BSSs A
and B.
Th oughpu gains come om mo e equen channel ac-
cesses, and he e o e a p opo ional educ ion on he channel
access delay should be also expec ed. Figu e 5b illus a es he
mean delay be ween wo consecu i e channel accesses o each
BSS. Wi hou NPCA, BSS A al e na es ansmissions wi h
BSS B, accessing he channel app oxima ely e e y 8.72 ms—
comp ising 4.36 ms o i s own ansmission and 4.36 ms o
BSS B’s. When NPCA is enabled, BSS A’s mean channel
access delay d ops o 2.95 ms, co esponding o a educ ion
ac o o app oxima ely 0.338. This imp o emen occu s be-
cause BSS A can access he channel mo e equen ly: ei he
by winning he con en ion agains BSS B o by le e aging
NPCA ansmissions, including pe o ming mul iple consec-
u i e NPCA TXOPs each ime BSS B accesses he channel,
p o ided ha BSS B’s ansmission du a ion allows i . This
explains why BSS A’s mean access delay alls below he
4.36 ms alue.
Highligh : NPCA imp o es h oughpu and eliabili y by
enabling access o seconda y channels when he p ima y is
occupied. O e lapping OBSS ansmissions—occupying only
he NPCA channel o he NPCA-enabled BSS— emain un-
a ec ed, as NPCA ope a es on di e en channels, ensu ing
no nega i e impac on hem. Channel access delay is p opo -
ionally educed as well, enabling as e and mo e equen
ansmissions.
B. A-MPDU size o Maximum NPCA gain
NPCA ansmissions bene i om longe OBSS ansmis-
sions. He e, conside ing Scena io I, we in es iga e how di -
e en A-MPDU sizes a ec NPCA h oughpu and channel
access delay gains. To his end, we e alua e se e al ixed
alues o he maximum A-MPDU size, deno ed as ∆. Ac-
co dingly, all ansmissions now include min(M, ∆) packe s,
whe e Mis he maximum numbe o packe s ha can i in
aTmax = 5 ms ansmission, depending on he employed
MCS and channel wid h. The dis ance be ween each AP and
i s associa ed s a ion is uni o mly selec ed a andom in each
scena io ins ance be ween 1 m and 17 m, as in he p e ious
sec ion.
Fig. 6a shows he h oughpu o BSS A wi h NPCA
disabled (blue) and enabled (beige). The NPCA gain, de-
ined as he a io be ween he h oughpu wi h and wi hou
NPCA, inc eases wi h ∆. This end esul s om longe OBSS
ansmissions by BSS B, which c ea e mo e oppo uni ies
o ex ended NPCA ansmissions by BSS A. The maximum
h oughpu NPCA gain is obse ed a ∆ = 128 packe s, whe e
he h oughpu nea ly doubles wi h NPCA enabled. This is
because BSS A can, on a e age, ansmi as many packe s in
i s NPCA ansmissions as i does du ing legacy 160 MHz
ansmissions. A lowe ∆ alues, NPCA o e heads limi he
h oughpu gain. A highe ∆ alues, he oppo uni ies o
NPCA a e cons ained by BSS B’s 80 MHz ansmissions
(a) Mean Th oughpu .
(b) Mean Channel Access Delay.
Fig. 6: Th oughpu and channel access delay in BSS A o
di e en maximum A-MPDU (∆) alues in Scena io I.
eaching he maximum TXOP du a ion (5 ms). In such cases,
while BSS A can include mo e packe s in i s 160 MHz
ansmissions, he h oughpu o i s NPCA ansmissions on
Ch#2 (80 MHz) becomes bounded by he same TXOP limi
as BSS B’s. The e o e, he highe h oughpu obse ed beyond
∆ = 128 packe s s ems exclusi ely om he 160 MHz
ansmissions, since NPCA ansmissions a e no longe able
o scale due o TXOP cons ain s.
Rega ding BSS A’s mean channel access delay (Fig. 6b),
i inc eases wi h he maximum A-MPDU size due o longe
ansmission du a ions. In e es ingly, he a io be ween he
legacy and NPCA delays emains nea ly cons an ac oss all
A-MPDU sizes, showing he same delay educ ion ac o
discussed ea lie in Sec ion V-A.
As o BSS B, al hough no shown, i s beha io ollows
he pa e n desc ibed in he p e ious sec ion. Inc easing he
maximum A-MPDU size esul s in bo h highe h oughpu
and highe channel access delay. When he A-MPDU size is
small enough o allow BSS B o ansmi all packe s wi hin he
TXOP, i s h oughpu ma ches ha o BSS A wi hou NPCA.
Howe e , once BSS B can no longe i as many packe s pe
TXOP, i s h oughpu d ops sligh ly below ha o BSS A
wi hou NPCA.
Highligh : The NPCA h oughpu gain is bounded by he
du a ion o OBSS ansmissions and is hus in luenced by
he A-MPDU size. Meanwhile, he channel access delay wi h
NPCA emains consis en ly a ound one- hi d o he baseline
ac oss all A-MPDU sizes, as NPCA enables nea -con inuous
ansmissions, o en allowing mul iple NPCA ansmissions
pe oppo uni y when OBSS ansmissions a e su icien ly
long.
9
(a) Mean Th oughpu .
(b) Channel Access Delay.
Fig. 7: Th oughpu and channel access delay when BSS A and
B use di e en MCSs, wi hou and wi h NPCA, in Scena io I.
C. O e coming he OBSS Pe o mance Anomaly
In he p e ious subsec ions, we analyzed mul iple ins ances
o Scena io I, whe e s a ions we e andomly deployed wi hin
he co e age a ea, esul ing in di e se MCSs. He e, we ocus
on speci ic cases whe e BSS A and B use ei he he lowes
MCS (MCS 1, BPSK 1/2) o he highes (MCS 11, 1024-QAM
5/6), wi h a maximum A-MPDU size o ∆ = 128 packe s. Ou
p ima y objec i e is o assess whe he NPCA can mi iga e he
OBSS 802.11 pe o mance anomaly—a phenomenon whe e
long OBSS ansmissions due o a low MCS deg ade he
h oughpu o all OBSSs, including hose using highe MCS
alues. This e ec was i s desc ibed in [20] o a single,
mul i- a e BSS.
Fig. 7a p esen s he mean h oughpu o BSS A and B,
wi h and wi hou NPCA, ac oss h ee MCS combina ions:
1) Bo h BSSs using MCS 11: Wi hou NPCA, BSS A
and B achie e equal h oughpu (490 Mbps) as bo h
ansmi 128 packe s pe channel access. Wi h NPCA,
BSS A le e ages i s seconda y 80 MHz channel (i.e., he
NPCA p ima y channel) du ing BSS B’s ansmissions,
inc easing i s h oughpu o 882 Mbps (×1.8 gain). As
expec ed, BSS B’s h oughpu emains he same.
2) BSS A using MCS 11, BSS B using MCS 1: Wi hou
NPCA, BSS A’s h oughpu d ops om 490 o 213 Mbps
because BSS B’s ansmission du a ion inc eases om
1.58 ms o 5 ms, educing BSS A’s channel access
a e. BSS B, despi e i s p olonged ansmission ime,
deli e s only 29 packe s, achie ing a low h oughpu
o 48 Mbps. Wi h NPCA, BSS A exploi s BSS B’s
ex ended ansmission pe iods o send mo e han 128
packe s ac oss mul iple consecu i e NPCA ansmis-
sions. Speci ically, a e comple ing an NPCA ansmis-
sion wi h 128 packe s ( he A-MPDU limi ), BSS A,
ecognizing ha i s p ima y channel is s ill occupied
by BSS B, ini ia es addi ional NPCA ansmissions as
desc ibed in Sec ion II. This p ocess epea s un il he
NPCA oppo uni y ends, boos ing BSS A’s h oughpu o
850 Mbps (×3.9 gain). These esul s highligh NPCA’s
e ec i eness in mi iga ing he nega i e impac o long
OBSS ansmissions.
3) BSS A using MCS 1, BSS B using MCS 11: Wi hou
NPCA, he oles a e e e sed—BSS A expe iences lim-
i ed h oughpu due o i s low MCS, which also penalizes
BSS B. Enabling NPCA does no yield any signi ican
gain, as BSS A is only able o ansmi six packe s du ing
he sho NPCA oppo uni ies.
Fig. 7b shows he mean channel access delay. When one
OBSS ope a es a a high MCS, i educes he channel access
delay o he o he , and ice e sa. These esul s a e consis en
wi h he h oughpu analysis, ein o cing NPCA’s e ec i eness
in add essing he OBSS pe o mance anomaly. I is wo h
no ing ha e en when no h oughpu gain is obse ed—such
as when he OBSS pe o ms sho ansmissions— he e is s ill
a clea educ ion in channel access delay. This bene i s low-
la ency sho ansmissions, which can ake ad an age o he
imp o ed channel a ailabili y.
Highligh : NPCA e ec i ely mi iga es he OBSS pe o mance
anomaly. When one OBSS ope a es a a low MCS, p olonged
ansmissions deg ade o e all ne wo k h oughpu . NPCA
enables high-MCS BSSs o exploi hese ex ended NPCA
oppo uni ies, signi ican ly imp o ing h oughpu .
D. Seconda y Channel Ac i i y: A Ze o-sum Game?
In his sec ion, we in es iga e he impac o OBSS ac i i y
on BSS A’s NPCA channel (Ch#2) in e ms o achie able
h oughpu , conside ing Scena io II, whe e BSSs A, B, and
D a e ac i e. Speci ically, we examine he e ec o a ying
BSS D’s ac i i y le els. To model his, we adjus BSS D’s
con en ion agg essi eness by scaling i s channel access a e λ
wi h he pa ame e αD, he eby con olling i s channel access
in ensi y (i.e., high (low) alues o αDco espond o low
(high) backo alues). We assume a maximum A-MPDU
size o ∆ = 128 packe s and andomize s a ion posi ions by
placing hem a dis ances uni o mly dis ibu ed be ween 1 and
17 me e s om hei co esponding AP.
In his Scena io II, when NPCA is no enabled, he asyn-
ch onous ope a ion among he BSSs signi ican ly limi s BSS
A’s abili y o u ilize he ull 160 MHz bandwid h. E en a
low alues o αD, Ch#2 is almos con inuously occupied
by BSS D, o cing BSS A o ansmi only on Ch#1. In
his con igu a ion, BSSs A and B ypically al e na e access
o Ch#1, while Ch#2 emains exclusi ely used by BSS D.
Howe e , when NPCA is enabled, BSS A gains addi ional
oppo uni ies o con end wi h BSS D. While BSS B occupies
Ch#1, BSS A swi ches o i s NPCA channel (Ch#2) and
a emp o access he medium he e, con ending wi h BSS D
as men ioned.
